GtkIconTheme: Rename a macro
GtkIconTheme: Rename a macro

We use DEFAULT_THEME_NAME in gtksettings.c as well, and this
can be a confusing when grepping, so rename this to
FALLBACK_ICON_THEME.

grubersjoe/adwaita,Adamovskiy/gtk,Lyude/gtk-,jessevdk/gtk,grubersjoe/adwaita,grubersjoe/adwaita,Lyude/gtk-,jadahl/gtk,Adamovskiy/gtk,chergert/gtk,jadahl/gtk,ahodesuka/gtk,chergert/gtk,Adamovskiy/gtk,jigpu/gtk,Lyude/gtk-,alexlarsson/gtk,alexlarsson/gtk,jadahl/gtk,alexlarsson/gtk,grubersjoe/adwaita,Adamovskiy/gtk,chergert/gtk,alexlarsson/gtk,Lyude/gtk-,ahodesuka/gtk,Lyude/gtk-,chergert/gtk,ahodesuka/gtk,ahodesuka/gtk,chergert/gtk,alexlarsson/gtk,grubersjoe/adwaita,grubersjoe/adwaita,jadahl/gtk,davidgumberg/gtk,davidgumberg/gtk,davidgumberg/gtk,ahodesuka/gtk,jigpu/gtk,grubersjoe/adwaita,chergert/gtk,jigpu/gtk,Lyude/gtk-,Lyude/gtk-,Adamovskiy/gtk,jigpu/gtk,jadahl/gtk,jigpu/gtk,Adamovskiy/gtk,jigpu/gtk,davidgumberg/gtk,jadahl/gtk,jessevdk/gtk,jessevdk/gtk,Lyude/gtk-,jadahl/gtk,chergert/gtk,jessevdk/gtk,ahodesuka/gtk,jadahl/gtk,jigpu/gtk,davidgumberg/gtk,davidgumberg/gtk,alexlarsson/gtk,grubersjoe/adwaita,chergert/gtk,ahodesuka/gtk,ahodesuka/gtk,davidgumberg/gtk,alexlarsson/gtk,jigpu/gtk,alexlarsson/gtk,jessevdk/gtk,jessevdk/gtk,Adamovskiy/gtk,jessevdk/gtk,Adamovskiy/gtk,davidgumberg/gtk